Some Field Strength Numbers

Since installing our EDM FM Transmitter, exactly like the one Tim in Bovey reviewed, set at minimum RF output power with a 1/4-wavelength horizontal Decade GPL32 dipole antenna, we've intended to make field strength measurements around the campus and today was the day.

The time was 11:30 AM, a TECSUN PL-310 with dBu/SN readings was the test instrument, its antenna open to 4-sections for a length of 13" always aligned horizontally on the same plane as the transmission, held at about 5' from the ground.

These numbers form a map of our grounds, all outdoors, X marks the transmitter, the top of the list is NORTH.

00/00 01/02 01/03

--/-- 00/02 --/--

--/-- 18/19 07/07

36/34 25/28 20/22

27/26 --/-- --/--

35/31 --/-- 27/30

40/31 --X-- --/--

52/41 53/49 43/43

--/-- 48/41

--/-- 42/31

--/-- 23/24

--/-- 05/06

The clear reception area on this radio is less than 100'.

The same clear reception area is observed on the auto radio in the driveway.

Weak spotty reception is detected on the auto radio in every direction for no more than 600-feet.

Resident Hobby Agent and the Part 15 Department of the FCC, I have to caution you that the use of non-calibrated Field Intensity Meters (portable radio sets) along with non-calibrated antennas does not provide assurance of compliance with applicable FCC regulations.

A biconical antenna is excellent when wide instantaneous bandwidth is needed, but for the most accurate field strength measurements, I would use a tuned dipole. In fact, tuned dipoles are sometimes used in EMC testing when a biconical shows the unit under test to be failing by a couple dB.

My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station.

KDX maintains two transmitters on its main frequency of 89.5 MHz, one is a backup should the other one be unavailable.

The EDM FM Transmitter was the first one tested in an earlier entry, now on to the Ramsey FM30b, also driving a Decade GPL32 Horizontal Dipole from a different location.

The two transmitters are much alike in audio quality and purity of the RF signals, but are different with regard to their RF output power levels: The EDM has two ranges, HI=1-10mW; LO=2-100mW. The FM30b has one range: 5uW-25mW.

For purposes of these measurements at 1:30 PM the FM 30b is set at its lowest 5uW output with the dipole=1/2Wavelength. Top of the list is North. X marks the transmitter. Numbers reflect dBu/SN from TECSUN PL-310 Radio, taken on the Internet Building Campus, home of KDX.

06/08 00/00 00/00

--/-- 07/10 00/01

--/-- 24/25 05/07

--/-- 29/33

34/36 36/37 32/34

44/44 --X-- 27/28

41/40

36/32 --/-- 27/28

21/23

24/25

--/-- 11/12 08/08

--/-- 10/09

--/-- 12/13

--/-- 06/07

Both of these transmitters were measured out to a distance of 100' and are easily well within the 200' allowed by FCC bulletin.

It is obvious from the numbers shown on my charts that I am well under the 200' typical range stated by FCC document for Part 15 FM.

Distance from these transmitters to the far boundry is right at 100' and we see that power is down in the zeros.

We made an interesting observation about this TECSUN PL-310 FM Radio.

It picks up a clear, listenable signal all the way down to a reading of 07 07, after which signal level drops off abruptly as the numbers reach 00 00, but even at that point the station can still be heard in the fizzle.

You are very right Brooce, the reflections happening within the signal field are there, but not possible to take into account with the limited kind of measurements we are taking.

Since our transmission antenna is a horizontal dipole we expect that the lobes are extending at right angles from the antenna leaving nulls at the tips of the dipole, but readings in the immediate field don't show much of a null on either side of the building. Maybe the null increases in size with distance, except we have no measurable signal on any distant path. And we can imagine that walls are bouncing the signal at angles which is probably very different from what would be experienced if the antenna were up high out in open space.

Today I happened to drive around and noticed that our FM signal becomes discernable right at the foot of the driveway, suggesting that our useful signal is within the confines of the property and virtually non-existant at any other property.

Following up on yesterday's power reading... 22 25 at the campus boundary, which is a public sidewalk... we wondered how much farther out we'd find the target minimum field reading of about 08 08, and we walked out 30-feet to the streetsign island that divides two confluencing streets, where the reading was 06 09, which equates with a reducewd signal submerged in FM fizz-noise.

Therefore 08 08 would be found in the main lane of the street and there's no audience there so the signal loses its meaning by being there.

With a "useful" signal range of 125-feet we are safely under the FCC's published allowance of 200-feet, so we'll stop here and make no more adjustments.

Besides using the TECSUN radio for our own compliance with FCC part 15, we have shown that these dBu/SN type radios CAN be used to setup for compliance.

We therefore now REQUIRE that all part 15 stations obtain and keep a radio with dBu and SM readouts. Deadline for compliance is 12 noon Decewmber 25th.

It may appear to some that many/most of the FCC §15.239 NOUOs are issued to operators of unlicensed systems using "high power" transmitters running tens or hundreds of watts, and that otherwise, there isn't much need be concerned.

But there are cases where NOUOs have been issued for systems radiating far less power (see graphic below).

Note that the radiated powers in the table apply only to free space fields. Signal reflections from the earth and other objects can either reinforce, or reduce them at some locations in the propagation environment.

Hobby Agent says: "There are cases where NOUOs have been issued for systems radiating far less power (see graphic). Signal reflections from the earth and other objects can either reinforce, or reduce them at some locations in the propagation environment."

While it is true that small numbers can be broken down into many many many smaller numbers into infinity, it is not clear whether this is a warning that an NOUO will be sent to KDX for being recievable to a drop-off distance of 125'.

It is not obvious whether the graphic table relates to the TECSUN line of radios with their dBu/SN indicators.

Possibly the posting does not address this thread in any particular way and should be given its own thread.

"Most S meters on traditional analog receivers are not calibrated and in practice can only provide a relative measure of signal strength based on the receiver's AGC voltage. Some S meters on traditional analog receivers are calibrated to read S9 for an input of -73 dBm but do not provide the correct 6 dB per S unit correspondence.Often the correlation between a radio listener's qualitative impression of signal strength and the actual strength of the received signal on an analog receiver is poor, because the receiver's AGC holds the audio output fairly constant despite changes in input signal strength."

DHR branches onto a path not previously taken in this thread about the field number readings on TECSUN-type radios.

Heretofore there has been no mention of "S" meters, but it's not unreasonable to take them under consideration as being less accurate signal-level indicators, in that they produce no numbers, other than the inaccurate number associated with "S9", as indicated.

However, to make use of both signal reading methods, that is by combining dBu indicators with "S" meters, a low power radio station expands its view of the RF field within the few feet being observed.

However that does not mean that the net field intensity arriving at the receive antenna equals 50 µV/m.

The reason for that (Carl), mostly is due to the unknown/uncalibrated characteristics of the receive antenna system at the received frequency.

A web-sourced definition of field intensity is "... electric fields created by electric charges, and by time-varying magnetic fields. The units of the electric field in the SI system are newtons per coulomb (N/C), or volts per meter (V/m)."

The electric fields of interest here exist between two different physical points in space -- not a conducted voltage existing between two terminals inside a radio receiver.

The FCC uses units of field intensity in several of its Part 15 rules applying to unlicensed use of the AM & FM broadcast bands. Examples are §15.209 for the AM band and §15.239 for the FM band.

As has been said: "The FCC uses units of field intensity in several of its Part 15 rules applying to unlicensed use of the AM & FM broadcast bands. Examples are §15.209 for the AM band and §15.239 for the FM band. Measuring such electric fields _accurately_ requires calibrated (expensive) test equipment used by a skilled and experienced operator."

And again, while it is good for the hobbyist to realize this in principle, the "calibrated expensive test equipment" that he will never own must be done without, leaving miniature radio stations with whatever make do solutions they can concoct.